A folded wavefront interferometer was insterted upstream of the imaging apparatus to measure the refractive index shift in the reduced density region surrounding the beam center. Gas density was extracted from retrieved interferograms using well-established fringe analysis techniques [1]}. For these measurements no fog was added to the chamber, as to prevent turbulence in the fog from interfering with the acquisition of the interferograms.
Figure REF (a) shows the variation of the refractive index for the off-resonant case, where the pulses in the train are detuned by 300 fs relative to each other, at a pump-probe delay of 500 \(\mu \) s. The reduced density region is visible at the prior location of the beam center, gradually expanding outward from this point. At the center, the relative refractive index shift is \(\Delta n_{\textit {off}}\approx -7.8\times 10^{-6}\) . If the pulse train is tuned to the revival time of molecular nitrogen, the reduced density region increases significantly in size and depth. At the same time delay of 500 \(\mu \) s, the resulting refractive index shift increases approximately by a factor of 2 to \(\Delta n_{\textit {on}}\approx -16.2\times 10^{-6}\)  (Fig. REF (b)). The asymmetry visible in the figure stems from convection [2]}. In the insets the filling in of the reduced density region can be observed by the smoothing of the lineouts over time. The lineouts depict the spatially resolved change in refractive index along the x-axis, centered on the minimum value at the core.
